The present invention relates to a toner substance for electrostatic photography and more particularly to a magnetic carrier composition constituting a component of the toner substance which enables a large number of copies to be produced by repeated developing and transfer from a single electrostatic image.
A toner substance for use in electrostatic photography in which a magnetic brush developing method is employed generally comprises fine particles of charge detecting composition mixed with fine particles of magnetic carrier composition. The charge detecting composition effects transfer of the toner substance from the magnetic brush to an electrostatic image of an original document which is formed on a photoconductive drum or belt and the magnetic carrier particles facilitate the formation of the magnetic brush.
A magnetic brush developing unit generally comprises an insulating cylinder disposed closely adjacent to the rotating photoconductive drum and one or magnets disposed inside the cylinder. The toner substance is applied to the cylinder and either one or both of the cylinder and magnet(s) are rotated so that the particles of the toner substance magnetically adhere to the cylinder in the form of a rotating magnetic brush of toner substance. The magnetic brush is maintained in brushing contact with the drum so that the toner particles adhere to the areas of the electrostatic image coresponding to the dark areas of the original document.
In many business and bureaucratic operations it is desirable to produce a large number of copies of a single original document. Since developing and transfer operations may be carried out at considerably higher speeds than an imaging operation, due to the photosensitivity of the photoconductive layer on the drum, it is desirable to perform the imaging operation only once, forming an electrostatic image on the drum, and to repeatedly develop and transfer from the image to produce a large number of copies at high speed.
However, a problem has heretofore remained in such a multiple copy process in that the alphanumeric characters and thinner line areas of the image progressively grow thinner as more copies are made, and eventually disappear. This has limited the number of copies which can be produced from a single electrostatic image to only 15-20.
It has been determined in accordance with the present invention that the cause of this detrimental effect lies in the magnetic carrier composition of the toner substance. Since the carrier composition generally consists of fine iron particles of ferromagnetic oxides, the conductivity thereof is quite high, causing discharge of the electrostatic image on the drum.
More particularly, such conductive carrier particles provide a discharge path for the high potential portions of the electrostatic image both to the magnetic brush and also to the low potential portions of the electrostatic image. This latter effect takes place most effectively at the edges of the high potential areas, thereby gradually eroding the edges as the developing operation is repeated. The effect is minimum in the central portions of solid black image areas, even though the edges of such areas are gradually eroded. However, thin lines and typewritten alphanumeric characters and symbols tend to become so thin as to disappear entirely after less than 20 copies are made.
In an attempt to eliminate this undesirable effect, toner substances have been introduced in which the magnetic carrier particles are coated with a highly insulating coating. Unfortunately, the results of using these toner substances appears to be about the same as with the simple iron carrier particles in the number of copies that can be produced due to thinning of image areas. In addition, the insulating coating is quite tough and tends to abrade the delicate photoconductive layer on the drum. However, the reason for the failure of the coated carrier particles is entirely different from that of the uncoated particles.
Due to the extremely low conductivity of the coated carrier particles, they are easily electrostatically charged and hold the charge for a long time. With the magnetic brush constantly sweeping toner substance in contact with the drum, many carrier particles contact high potential areas of the electrostatic image, become electrostacally charged thereby removing a corresponding amount of charge from the electrostatic image, and subsequently contact low potential areas of the electrostatic image to become discharged. In this manner, charge is effectively removed from the electrostatic image by the carrier particles. The effect, as in the case of the uncoated carrier particles, is most noticable in the thin areas of the image.